Finding sources relevant to your question should not depend solely on database searching. Supplementary search methods are recommended in order to mitigate publication bias (the practice of publishing studies with positive effects and rejecting studies with zero or negative effects). Add this step to your methodology to ensure that you've taken steps to go beyond the formal structure of databases, including the inclusion decisions made by their publishers.
Review works cited lists from similar reviews and from the articles that have met your eligibility criteria.
Hand Search Specific Journals
Manually browse through journals that often publish studies of interest to your review. Check journal coverage in your chosen databases to see what years might not be included in your systematic search and then hand search those issues for relevant articles.
Not all clinical trials end in publications. Clinical Trial Registries help you locate otherwise unpublished or incomplete trials.
Dissertations & Theses
Reports and conference papers
Look for conference proceedings from relevant conferences and associations. Scan publication lists from related research centers and institutes.
Though Google and Google Scholar searches will not be part of your systematic methods, Google can be used to find publications from organizations, centers, and institutes of interest. Google Scholar may help turn up additional grey literature (see Document Types in Grey Literature) and some journal articles that may have been missed in the systematic search. See also: Gray Literature Research Guide (from the University of Michigan)
Google Scholar is also a valuable tool for finding articles that are too new or otherwise not indexed in your chosen databases. Use a more simplified version of your search strategy and comb the first few pages of results. You can also do a search that limits results to the last two years so that you can make sure you've captured the latest publications.
Reach out to scholars who have conducted research on your topic and solicit publications or data that may not have been formally published.
This guide was adapted from Systematic Reviews by University of Texas Libraries (https://guides.lib.utexas.edu/systematicreviews) which is licensed under a Creative Commons Attribution-NonCommercial 2.0 Generic License.